The present invention relates to an optical record-reproduction device, and, more particularly to an optical pickup device which can be suitably used in an magneto-optical disc device capable of recording/ reproducing/erasing.
Optical discs capable of recording/reproducing of information or recording/reproducing/erasing of information by the converging of a beam from a semiconductor laser on small spots through an object lens have recently attracted great attention. The reason for this lies in the fact that optical discs possess a large capacity and are inexpensive in terms of cost per bit. In addition, since these discs are designed to be removed, handling is easy for users.
However, optical discs suffer from the disadvantage that access time is too long compared with conventional hard discs such as magnetic discs. When recording is performed with an optical disc, an address is first accessed so that a spot may be positioned at a desired address in response to the recording command. The recording is then performed, and validation and reproduction of written information may be carried out after the recording has been completed. Since the recording mode of the optical discs thus comprises two modes, that is, the recording mode and the validation and reproduction mode, a considerably long time is taken to complete recording.
In order to overcome the above-described disadvantage, a multibeam optical pickup device has been proposed which is arranged in such a manner that a plurality of beams are successively converged on the same track so that a preceding beam is used for recording information and an ensuing beam is used for validating and reproduction (RAW). The system of the type described above offers the advantage that recording and reproduction can be simultaneously performed during one rotation, whereby the time taken to complete the recording mode can be shortened.
The system was, for example, reported in "Trial Manufacturing of 3.5-inch small magneto-optical disc device" one of the theses presented in prior to the Spring National Meeting of the Electronic Information Communication Society held in 1988.
However, a conventional multibeam optical pickup device of this type arranged in such a manner that a semiconductor array or a plurality of semiconductor lasers are used as the light source and an image is formed on a disc through the same object lens so as to obtain a plurality of spots involves certain disadvantages in that the relative positional relationship between spots on the disc is determined by the characteristics of the employed semiconductor laser (intervals of the chips and wave length) and that such a semiconductor laser which includes a plurality of the light emitting points is very expensive.
Systems for multibeam optical magnetic pickups are conventionally classified as follows: systems comprising a plurality of light sources in which individual light sources are used; and LD array systems in which a semiconductor laser array including a plurality of light sources is used.
With a multibeam magneto-optical pickup, beams from a plurality of light sources are individually converged through the same object lens on the same track of the magneto-optical recording medium. However, in order to simultaneously focus a plurality of beams on the magneto-optical recording medium, it is necessary to perfect a greatly improved degree of positional accuracy for each of the light sources. Therefore, positioning and mounting of the light source becomes extremely difficult.
On the other hand, in the LD array system, a problem arises in that a minimum distance between the spots of the corresponding beams that are converged on the track must be determined by the LD array. The reason for this lies in the fact that any reduction in distance between the light sources is to a certain degree limited because it is necessary to prevent relative affection caused by the change in the temperature of chips. In addition, in order to prevent creation of unequal distances between the spots, the distance between the chips of the LD array needs to be measured. As a result, the number of manufacturing processes required to assemble an magneto-optical pickup is increased and it is therefore difficult to reduce the cost of production thereof.
On the other hand, as is known, the oscillating wave length of a semiconductor laser may change due to variations in temperature. In an magneto-optical pickup in which an LD array is used as the light source, each semiconductor laser has an individual light source and thus does not emit light with the same output power. Each of the light sources thus has its own individual oscillating wave length. As a result, the converging position of each of the light beams from the object lens will inevitably be shifted in the direction of the optical axis due to the chromatic aberration of the optical system. Even if a dual wave length HLD array is used as the light source, the wave length of each of the light sources is originally different from the others, which also leads to the above-described problem due to the chromatic aberration generated. It might be considered feasible to employ an achromatic lens in the optical system. However, this involves the drawback that the overall cost of the optical pickup would be excessive because achromatic lenses are expensive.